Orthophosphate and phytate extraction from soil components by common soil phosphorus tests

Chemically extractable phosphorus (P) is widely used for environmental risk assessment, because of its strong relationship with P loss through surface runoff and/or subsurface leaching. We used five common soil test extractants, water, 0.01 M CaCl2, Mehlich I, Mehlich III and acidified ammonium oxalate in darkness (AOD) to recover two major P forms, orthophosphate (Pi) and phytate (inositol hexakisphosphate or IHP), which were adsorbed at various P saturation levels on Na-saturated goethite, gibbsite, kaolinite and montmorillonite. Our objective was to evaluate how the P form and mineral type influence the efficiency of these extractants to solubilize P and to examine the inter-correlations between extractable P fractions and the implications in P risk assessment. Phytate had a greater Langmuir adsorption maximum than Pi for all the studied minerals, whereas IHP had a lower extractability than Pi by all extractants. Phosphorus extractabilities increased in the order goethite < gibbsite < kaolinite < montmorillonite. Water and 0.01 M CaCl2 extracted negligible IHP from all P saturation levels of these minerals. Mehlich III and AOD were equivalent in terms of the amount of P extracted. Mehlich III-P showed the best prediction of AOD-P, which is used for estimating soil degree of P saturation (DPS), whereas Mehlich I had a close relationship with AOD-P within a P form. The relationship between DPS estimated by AOD extraction and the true surface P saturation and the factors affecting the relationship were discussed in the model systems.